1. Field of the Invention
The present invention relates to an electron gun for a color cathode-ray tube, and more particularly to a configuration of electrodes of an electron gun constituting the main electrostatic focusing lens of the electron gun.
2. Description of the Prior Art
A conventional color cathode-ray tube with an in-line type electron gun is shown in FIG. 1. A glass envelope 1 of the tube is composed of a front panel 2 and a funnel 3 connected to the panel 2. A fluorescent screen which is coated with three color phosphors for developing a color image is disposed on the inner wall of the panel 2. A shadow mask 4 for color selection is disposed inside the envelop 1 adjacent the panel 2 and spaced from the fluorescent screen.
An electron gun 5 is coaxially disposed inside the tubular neck portion 3a of the funnel 3 to generate and direct three electron beams, which represent the three primary colors respectively, along coplanar convergent paths through the shadow mask 4 to the fluorescent screen. More specifically, the electron beams, which are composed of thermions, are emitted from cathodes 6, 7 and 8 of electron gun 5, and pass through corresponding apertures in first and second grid electrodes 9 and 10. Then the electron beams are directed along the electron beam paths 11, 12 and 13 (shown at the solid lines of FIG. 1) to the front 2, respectively. At this time, each of the cathodes 6, 7 and 8 and its corresponding aperture formed in the first and second grid electrodes 9 and 10 have a common central axis parallel to the others on a common plane. The three central axes are coincident with the electron beam paths 11, 12 and 13, respectively.
Referring to FIG. 1, the line Z--Z extending along the central electron beam path 12, i.e., the center of the electron beam paths 11, 12 and 13, to the panel 2 is called the "axial direction", hereinafter. Similarly, the line X--X, which is perpendicular to the axial direction and extending across the common plane including the electron beam paths 11, 12 and 13, is called the "horizontal direction". The line Y--Y (not shown), which is perpendicular to the axial and horizontal directions, is called the "vertical direction."
Thereafter, the three electron beams travel through the first and second grid electrodes 9 and 10 along the electron beam paths 11, 12 and 13 arranged on the common plane and then travel through third and fourth grid electrodes 14 and 15. Here, the third and fourth grid electrodes 14 and 15 constitute an auxiliary focusing lens or a pre-focus lens. Then, the electron beams travel through a focus electrode 16 (hereinbelow, referred to as "the first accelerating/focusing electrode") as well as an anode electrode 17 (hereinbelow, referred to as "the second accelerating/focusing electrode"), both electrodes 16 and 17 constituting the main focusing lens of the electron gun 5.
To constitute the main focusing lens, a potential of 25 KV-35 KV is applied to the second accelerating/focusing electrode 17, and a potential of about 20%-30% of that applied to the second electrode 17 is applied to the first electrode 16.
Since the center portion of the main focusing lens, which is formed by the potential difference between the first and second accelerating/focusing electrodes 16 and 17, is coaxial with the central electron beam path 12, the central beam, i.e., one center beam of the three electron beams, which travels through the center portion of the main focusing lens is focused to be thin and accelerated to travel straight along the axial direction to the fluorescent screen.
However, since the outer portions of the main focusing lens are not coaxial with the central electron beam path 12, two outer beams of the three electron beams which travel through the outer portions of the main focusing lens are not only focused to be thin, but also subjected to a converging effect toward the central electron beam.
Hence, the three electron beams are converged onto the shadow mask 4 in an overlapping fashion and then accelerated to reach the fluorescent screen, thus to form a spot on the screen.
To scan electron beams on the fluorescent screen, an external magnetic deflection yoke 18 is externally provided adjacent glass envelope 1. The above operation for thinning electron beams by the main focusing lens is called "focusing" and the above operation for converging electron beams by the main focusing lens is called "static convergence (hereinbelow, referred to simply as "the STC")".
FIG. 2 illustrates a partially cutaway perspective view of the first and second accelerating/focusing electrodes 16 and 17 constituting the main focusing lens of the prior art electron gun of FIG. 1. The first accelerating/focusing electrode 16 is composed of a non-cylindrical electrode tube with one end open and another end partially closed.
The first electrode 16 includes an envelope 19 from which a closed end face 20 extends. The closed end face 20 includes three separate beam passage apertures 41, 42, and 43 which are axially parallel to one another. These beam passage apertures 41, 42 and 43 are surrounded by cylindrical lips 51, 52 and 53, respectively. Each cylindrical lip is projected from the closed end face 20 inwardly toward the open end of the envelope 19.
Second accelerating/focusing electrode 17 has substantially the same configuration as the first electrode 16 and is symmetrical to the first electrode 16 with respect to the horizontal direction. This second accelerating/focusing electrode 17 includes an envelope 21 and a closed end face 22 integrally formed with the envelope 21. The closed end face 22 includes three electron beam passage apertures 61, 62, and 63. These beam passage apertures 61, 62 and 63 are surrounded by individual cylindrical lips 71, 72 and 73. Each cylindrical lip is projected from closed end face 22 inwardly toward the open end of envelope
The outer beam passage apertures 41 and 43 of the first accelerating/focusing electrode 16 are spaced apart from the central beam passage aperture 42 at an equal first distance, i.e., the center to center distance, along the horizontal direction and this distance is equal to the distance between each of the outer electron beam paths 11 and 13 and the central electron beam path 12 of FIG. 1. Likewise, the outer beam passage apertures 61 and 63 of the second accelerating/focusing electrode 17 are spaced apart from the central beam passage aperture 62 at an equal second distance. The second distance is slightly greater than the above first distance.
These first and second accelerating/focusing electrodes 16 and 17 are arranged such that their closed end faces 20 and 22 face each ether and are spaced out at a given distance "g".
In accordance with this prior art configuration, the three separate main focusing lenses are provided for the three electron beams, respectively.
Otherwise stated, three pairs of electron beam passage apertures, a first pair consisting of the outer apertures 41 and 61, a second pair of the central apertures 42 and 62 and a third pair of the outer apertures 43 and 63, are provided in first and second accelerating/focusing electrodes 16 and 17. The above three pairs of electron beam passage apertures are surrounded by three pairs of lips, that is, a first pair consisting of lips 51 and 71, a second pair of lips 52 and 72, and a third pair of lips 53 and 73, respectively. Here, the three pairs of electron beam passage apertures 41 and 61, 42 and 62, and 43 and 63 constitute the three separate main focusing lenses, respectively, each of the three focusing lenses focusing a respective one of the three electron beams.
As described above, the second distance between the beam passage apertures 61, 62 and 63 of the second accelerating/focusing electrode 17 is greater than the first distance between the beam passage apertures 41, 42, and 43 of the first accelerating/focusing electrode 16. Thus, the central main focusing lens, which includes the central beam passage apertures 42 and 62, is coaxial with respect to the axial direction, while the other main focusing lenses or the outer main focusing lenses, one including the outer beam passage apertures 41 and 61 and the other a pair of the outer beam passage apertures 43 and 63, are not coaxial with respect to the axial direction.
Accordingly, the central electron beam passing through the central main focusing lens is focused to be thin and accelerated to travel straight along the axial direction to the fluorescent screen, while the outer electron beams passing through the outer main focusing lenses are not only focused to be thin, but also subjected to a converging effect toward the central electron beam.
However, as apparent to those skilled in the art, the known electron gun is apt to be affected adversely by spherical aberration of its main focusing lenses since its main focusing lenses have small apertures of about 5.5-5.9 mm. In this regard, there occurs a haze phenomenon in that the peripheral light of an electron beam spot is not clear, and this deteriorates the resolution of a color cathode-ray tube.
This haze phenomenon is noted to be mainly affected by the aperture R of the main focusing lens.
That is, the spherical aberration of the main focusing lens is in inverse proportion to R.sup.3 or the third power of the aperture R of the main focusing lens, and this aperture R is substantially proportional to a diameter D of corresponding electron beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17.
It is thus preferred to enlarge the diameter D of the electron beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17 in order to reduce the bad effect given by the spherical aberration of the main focusing lenses.
However, such an enlargement of the diameter D is also accompanied with deterioration of lens action of the main focusing lens, and thus results in a flying spot aberration in that the focusing voltages for the electron beam spots do not precisely agree with each other.
If described in detail, the Z-axial potential function .phi."(Z) and the spherical aberration component C are represented by the following relations (I) and (II), respectively. EQU .phi."(Z).varies.2/g.times.(V.sub.2 -V.sub.1).times.1/R (I) EQU C.varies.M/(16.times.R.sup.3) (II)
wherein
V.sub.1 : voltage of the first accelerating/focusing electrode 16, PA1 V.sub.2 : voltage of the second accelerating/focusing electrode 17, PA1 g: distance between the first and second accelerating/focusing electrodes 16 and 17, PA1 M: magnification of the main focusing lens, and PA1 R: aperture of the main focusing lens. PA1 Dsc: enlargement component of the electron beam by a space charge effect, that is, Dsc=f(rsc/ri).varies.(i.sup.1/2 /V3/4)(Z/ri) wherein i is a flying beam, V is a high voltage, and rsc/ri is the beam spread, and PA1 Dsa: enlargement component of the electron beam by the spherical aberration component. PA1 D: diameter of each of the beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17, PA1 S: beam separation, PA1 G: minimum gap between the first and second accelerating/focusing electrodes 16 and 17 and the inner surface of the neck portion 3a, the gap allowing electric insulation to be achieved between the electrodes 16 and 17 and the inner surface of the portion 3a, and
In accordance, with the above when the aperture of the main focusing lens is enlarged by .delta.R, the lens action of this main focusing lens is reduced by about 1/.delta.R and, C (the spherical aberration component).apprxeq.1/(.delta.R).sup.3.
When the aperture R of the main focusing lens is enlarged in order to remove the problem caused by the flying spot aberration as described, the size Ds of the electron beam spot on the screen is represented by the following relation (III). ##EQU1## wherein Dx: enlargement component of a cross-over point dx by the magnification M of the main focusing lens, otherwise stated, Dx.varies.(Midx,)Mxdx,
However, since the three electron beam passage apertures of the in-line type color cathode-ray tube are arranged on a common plane as described above, the beam passage apertures 41, 42 and 43 of the first accelerating/focusing electrode 16 and the beam passage apertures 61, 62 and 63 of the second accelerating/focusing electrode 17 are limited in their diameters to be not more than 1/3 of an inner diameter of the neck portion 3a of the cathode-ray tube.
As described in detail in conjunction with FIG. 3, the inner diameter L of the neck portion 3a of the cathode-ray tube is represented by the following relation (IV). EQU D+2(S+G+1).ltoreq.L (IV)
wherein
l.sub.1 and l.sub.2 : bridge widths between the beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17, the widths being minimum widths allowing the electrodes to be mechanically prepared.
At this time, each of l.sub.1 and l.sub.2 should be longer than 1.0 mm from the viewpoint of the conventional designing condition of the electrodes 16 and 17, and thus this results in D.gtoreq.Z S-1 (mm).
In addition, the gap G between the envelopes 19 and 21 of the first and second accelerating/focusing electrodes 16 and 17 and the inner surface of the neck portion 3a should be longer than 1.0 mm such that electric insulation is achieved between the electrodes 16 and 17 and the inner surface of the neck portion 3a, and this results in D.ltoreq.(L/3)-2 (mm).
Accordingly, the diameter D of each of the beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17 is inevitably limited to be not more than 1/3 of the inner diameter L of the neck portion 3a of the cathode-ray tube.
However, in the prior art electron gun, the enlargement of the diameter D of the electron beam passage apertures of the first and second accelerating/focusing electrodes 16 and 17 is achieved only by enlarging either the beam separation S or the inner diameter L of the neck portion 3a.
Thus, the prior art electron gun has a problem in that it increases electric power consumption for the external magnetic deflection yoke 18, and deteriorates the beam converging effect toward the central electron beam due to the enlargement of the beam separation.